: Bronchopulmonary dysplasia is a complication of prolonged mechanical ventilation after premature birth. To study the pathophysiology of this disease, the investigator has developed an animal model of BPD, following premature delivery of lambs, and 3 weeks of mechanical ventilation. Utilizing this model, physiologic, biochemical, histologic, and molecular techniques will be used to determine (1) if incomplete lung development is essential for the vascular and structural abnormalities of the pulmonary circulation that occur in chronic lung disease, (2) if inhaled nitric oxide will enhance the NO-cGMP cascade and facilitate postnatal regression of vascular smooth muscle in these lambs, and (3) if decreased availability of L-arginine and/or increased activity of cGMP-specific PDE contributes to the sustained elevation of PVR and the abnormal postnatal regression of vascular smooth muscle in these premature lambs.